CSS Specificity
Specificity in CSS is a set of rules to define how the order in which styling is displayed. The browser has default styling rules. These rules are overridden bt the user, and the developer. Different componants have weight. Inline CSS has more weight than ID's and classes and so on. There is a way for a user to set something like text size to very large so that people who are sight impaired can override the developers styling.
Contrast Ratios
Contrast ratios are the difference between the text color and the background color. Some color combinations may look nice together, but when they are too similar in tone and brightness they begin to blend together.
Why You Must Meet Contrast Ratios
It is important to get contrast ration within specifications so that your content can be read by any visiter to the website. People with poor vision need a higher contrast to decifer the text from the background. If a company's site is not accesable to everyone they open themselves up to potential law suits. Being assessable to everyone drives more visiters to your site and not only protects you from law suits, but more visiters also also could mean more money in your pocket.
Who Sets Accessibility Rules
The World Wide Web Consortium (W3C) sets the assessibility standards. The standards are reviewed by the Accessible Platform Archetecture Group (APA).
Summary
Knowing the order of specificity and the rules about accessibility helps developers create sites that are user friendly and do not set your business up for a law suit.